Q: What would you experience if you were going the speed of light?

The original question was: If I’m moving at the speed of light towards you and I throw a tennis ball at you (at, say, 5 m/s), what do you observe? Will I hit you first, the tennis ball, or both at the same time, or will something else happen entirely? The problem that I’m having thinking about this is that if the tennis ball did hit you first, then it would have been moving faster than the speed of light (relative to you). On the other hand, if it were moving at the speed of light relative to you, then it was moving at the same speed as me relative to you, thus both myself and the tennis ball will hit you simultaneously. If this is the case, however, then the tennis ball would have been moving at a speed of 0 m/s with respect to me.

Mathematician: First of all, let me point out that you will never travel at the speed of light (see this for details). It would take an infinite amount of energy to get anything with mass (e.g. you or your mama) going at that speed. Burning all the oil (and plants, and animals) on our planet and converting them into kinetic energy would get you going really fast, but would give you exactly 0% of the total energy that you would actually need to get going that speed (since any number divided by infinity is zero). But that doesn’t mean that it wouldn’t be fun to speculate about what would happen if you were going at light speed.

Due to a relativistic effect known as time dilation, the faster that you move with respect to some object O, the more time slows down for the object O (from your perspective). This isn’t just an issue of you seeing clocks attached to O tick slowly, they actually DO tick slowly from your vantage point (no experiment you could possibly do would conclude otherwise). There is symmetry though. You moving past O at 10,000 miles per hour is indistinguishable (as far as the laws of physics are concerned) from O moving past you at 10,000 miles per hour. That means, from the perspective of a person strapped to O, clocks attached to you are ticking slowly (i.e. your time is slowed down). As you approach the speed of light (with respect to O), this time dilation effect becomes more and more pronounced. When you are exactly AT the speed of light (impossible, but bare with me) no time whatsoever will elapse for O (i.e. a clock strapped to O will stop ticking completely) from your perspective. The upshot of this is that you’ll get wherever you are going without witnessing any time pass for anything not moving along with you. One reason this is really trippy is because if we view light coming from a distance object (such as a far away sun), from our perspective it might have taken years to get from us. But from the perspective of the photon (i.e. the light particle) no time will have elapsed on the journey! Yes, true physics is even weirder than crazy person made up physics.

A possibly even wackier effect crops up as the result of length contraction (another consequence of relativity). If you move towards object O at a fast speed you will notice that O will be compressed (i.e. shrunk) along the direction of your motion. So if O is a hippo, and you are going fast enough, it will look like one damn flat hippo.

Um, something like this I guess?

As you approach the speed of light, this effect becomes increasingly pronounced, and at at the speed of light itself O will have zero length in the direction you are traveling. In particular, if you are on a straight race track, and traveling at the speed of light, the race track will be compressed to zero length so that the starting line and the finishing line will be on top of each other. The race will be over as soon as it begins.

Another consequence of light speed travel is that you’d become the most dangerous thing imaginable (move over, Chuck). Since your mass is positive, infinite speed implies that your momentum is infinite. Hence, if you crashed into anything (and you would…after all, from your perspective the universe is flatter than a pancake in the direction you’re heading) it would get hurtled at insane speeds (since it would absorb some of your momentum). Of course, you’d also be dead pretty much instantly as you collided with object after object (each traveling at the speed of light with respect to you). And no, armor wouldn’t help.

Okay, so now to address the original question. What would happen if while traveling at the speed of light towards me you attempted to throw a ball at me? The answer is that you would have no time to actually do the throwing, because from your perspective you would run into me instantly. At which point, if I had any ninja skills, I would probably break those out. If you were traveling at near the speed of light (with respect to me), but not quite at it, and then threw the ball at 5 m/s (with respect to you) in my direction, the velocities would not simply add like you would expect based on Newtonian mechanics. Instead, you’d have to apply relativistic velocity addition which is a bit more complicated. In particular, the speed of the ball with respect to me will be less than the sum of your speed and 5 m/s. At low speeds this effect is not noticeable (speeds are additive to very close approximation), but at speeds close to the speed of light the effect becomes very pronounced.

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39 Responses to Q: What would you experience if you were going the speed of light?

  1. david newman md says:

    you mentioned length contraction as velocity increases toward the speed of light. is there any connection between a flat race track and a flat universe (two dimesional)?

  2. The Physicist Physicist says:

    Also, are you talking about horse race tracks or what?

  3. david newman md says:

    race tracks and everything.. but how can anyone walk around in a flat universe boy, talk about low clearance! like in being john malkovitch whee the office they worked in was five feet high

  4. I understand (or don’t I) that as you approach the speed of light objects become so massive that no energy would be capable of moving them, is that right?

    Then, why we don´t see this effect in reality as photons travel at the speed of light?

    Are photons real? Light could not be transmitted by waves like sound because in space there is no “medium” for the waves to travel?


  5. Bo Brymer says:

    A blur of energy, color and light

  6. John says:

    I just wanted to ask this one question because I feel like I understand most ‘simple’ things that have been posted around the internet about special relativity but there is one thing bugging me:
    If a body is travelling very close to the speed of light and the time on board for them has slowed, what would a person on the outside of them — looking to where they would be — see?
    Would they see anything? Or would they see them in a slow-motion like way because of time dilation? I’m confused about how a person on the outside would see them or whether they would be able to see them at all because they’re travelling at the speed of light after all. Please talk in a completely realistic way. Could a person actually see anything? Or would you have to presume that the person had been squished in the direction of travel?

    Sorry for the amount of question, but I hope you get the idea.

    Thanks, John.

  7. Jnaneshwara G K says:

    If a object travel with a speed of light, then it looks like a rays where you cannot recognize the real object travelling. ex. If a superman or spiderman travelling with speed of light you will not recognize who they are? so you need a superfast camera to detect them in that speed, most probable…900000000000M fixel camera or more powerfull? So If such fast object hits a earth or having a big mass what may be result of it? so how we can detect a speeding photol or electron ?

    Jnaneshwara G K

  8. The Physicist The Physicist says:

    You let it hit something. Trying to take a picture isn’t worth the bother.

  9. Jonathan says:

    The thing is, if you were traveling at the speed of light wouldn’t you not be able to move forward? Meaning the signals in your brain would not be able to move. This would also apply to signals in a computer chip. And if you can’t move forward, you can only move backward, so technically wouldn’t you be dead?. Also, if you were sitting on something (such as a chair), you would vibrate into it.

  10. David says:

    As far as effects go. The simple act of moving through air particles at a rate of speed not even vaguely near the speed of light, the friction would obliterate you and possible cause a nasty combustion/force wave in the surrounding area.
    I suspect that at that immense of a speed, air would almost act as a solid in terms of friction.

  11. Ismael says:

    In regards this I thought your mass would increase and theoretically once you have more mass than object around you, gravity on you increases, thereby you draw other object from around you towards you.

    If and hopefully not you cannot support said mass, your gravity would create a black hole in you, imploding you almost instantly.

    Hence unless you can have infinite energy to counteract your infinite mass, you die.

    Also as David mentioned, and always bothered me though your description of space getting squeezed managed to confuse me more/ clarify a bit, does this mean things like air have no effect on said body as I thought you would experience some resistance of some sorts which would lead to friction and at that speed should also mean death?

  12. Justin Kyllo says:

    As you approach the speed of light your mass is infinite and your dimension approaches zero, meaning when/if you collided with anything it would simply be just protons/neutrons, but with your unlimited mass vs their super small mass it would be like a fly hitting you.

  13. ED says:

    What I don’t understand is a person travelling close to the speed of light, does his biological clock slow down? How does he stay younger on his return?

  14. Glenn Shrom says:

    If I am traveling at 0.9 c, and I observe object O accelerate from 0.5 c to 0.8 c, am I seeing the opposite happen for O from what we usually say happens as something approaches the speed of light? Am I seeing the mass of O decrease as it accelerates, time speeding up for O, etc.? When we normally talking about something approaching the speed of light, we normally talk about it from the perspective of an observer that is traveling slower than that object. But would the same changes be observed if we are going faster than it while it accelerates?

  15. The Physicist The Physicist says:

    The object starts out moving at 0.4c* and it ends up moving at 0.1c*. So, the relativistic effects decrease. Someone with a different perspective will describe the situation differently (but consistently). After all: relativistic effects are relative.

    *There are some subtleties involving how speeds are combined in relativity, so someone moving at 0.9c seeing something moving at 0.8c won’t see it moving at 0.1c, but the idea is the important thing here.

  16. Paradigm says:

    I’m very confused. This (http://science.howstuffworks.com/science-vs-myth/what-if/what-if-faster-than-speed-of-light1.htm) article says that time dilation makes time move slower for objects moving very fast. eg. A clock on a space ship would advance less than a clock on a planet it passes by. Therefor if you looked out the window at the people below they would be walking around in fast motion as thier clocks speed ahead of yours. Also to them you would be moving in slow motion in your spaceship. The faster you travel the slower your time passes and the slower you can move. At the speed of light you would be frozen completely and forever.

    We know that stronger gravitation slows down time, so it fits in well that infinate mass at the speed of light could contribute to the time freeze.

    But you seem to be saying that time on the planet below actually slows down relative to you, and for people on the planet looking into your spaceship, your time also slows down to them. So is that other article wrong or am I missing something? Is it different if the two objects have vastly different mass, like a ship vs a planet?

    Suppose nothing exists in the universe except for two similar spaceships with crews and a clock, and both sets sum up to the same mass. You’re telling me that if one ship moves really fast towards the other one, then they are both moving. I can understand that.
    But then you say that if one crew looks at the other crew’s clock they will see it going slower than theirs, and the same happens for the other crew. So when they meet and compair clocks who’s clock will be ahead or behind?

    We know from experements that clocks on moving vehicles move slower. (http://www.alternativephysics.org/book/TimeDilationExperiments.htm) Therefore I think that the time in the reference frame of the moving object is slowed while time outiside of that reference frame is relatively sped up.

    I think that in the case of the two ships, the one that is pushed gains more mass, gravity, energy, etc which maybe cause its time to slow down relative to the other ship which although apparantly moving relatively at the same speed, gains no energy, and therefore should really be considered as non-moving. Besides, the non-moving ship is also motionless relative to the vacume/darkmatter where as the pushed ship is moving relative to the vacume, darkmatter, and the other ship.

  17. The Physicist The Physicist says:

    It’s deeply weird, but the situation you’re describing (where both ships see the other ship’s clocks running slow) is exactly the case.
    There’s an old post here that tries to explain that, and another here that talks about the twin paradox which is related, but very different.

  18. Ed says:

    What I would like to know is what happens to us biologically to stay younger when we travel near the speed of light? Do our cells slow down too?

  19. Matt says:

    @Ed, I believe that it’s less of anything happening to us biologically and more everything around you happening much quicker. From reading through this, if one were to hit the speed of light then time would basically stop for that person. So from that state of time being stopped, while we would age relative to Earth time (or whatever non-lightspeed object you wish to compare it to) our bodies would not actually be doing anything since time would have essentially stopped for us.

    This is a layman’s view so correct me if I’m wrong.

  20. Ed says:

    Thank you Matt for the answer. However, after returning from near speed of light journey to Earth, there is an age difference between the traveler and the observer on Earth. If there is an age difference, is there a biological difference too? If yes, how?

  21. zeke says:

    So, I’ve been reading, and I understand that there is not enough energy for me to travel at the speed of light, but is there enough energy in the universe for the universe to zip past me at the speed of light? …or is that WHY it would take infinite energy for me to go that fast?

  22. michael says:

    Time would pass normally for you but everyone else would be moving much faster and age quicker. It doesn’t “keep you younger longer” in the way you might want it to… It doesn’t make time FEEL slower for you. It just makes time pass much faster for everyone else.

  23. West says:

    One question…if a photon experiences no time, then is any given photon everywhere at once with respect to itself? For example, if a photon from a light source, like a star, traveled to a mirror one light year away, bounced off and then traveled to another mirror another light year away, how could it experience both mirrors simultaneously?
    I know it “traveled” only in respect to us observing it…but if a photon is everywhere at once, and arrives instantaneously, then aren’t all photons everywhere?


    Does the biological clock too slows down when one goes to space? If yes, how is that possible, does the concept of Time Dilation applies to the Biological system too.

  25. Michael MacDonald says:

    So, if you were travelling at the speed of light, and didn’t hit anything, but decided to go 100,000 light years, you would get there instantaneously. However, the people at home would all have died almost 100,000 years ago. Unless they followed you there, and didn’t hit anything either.

  26. Ed says:

    Why do we spend so much time analysing something which is impossible? We can never travel at the speed of light. It is not the technology that is preventing us from reaching the speed of light, it is the nature not allowing us.

  27. tom says:

    So inorder for someone to travel at the speed of light that person should have unlimited energy inoder to move the amount of mass in his body .if you are traveling the speed of light in a plannet like earth but with strong wind currents opposite the velocity he is traveling at will the be any resistance

  28. Nkosi says:

    And I quote, “at the speed of light the mass of his spacecraft would become infinite, its length would shrink to nothing and time aboard it would slow to a complete stop. Since this is impossible nothing can travel faster than light.”

    But lets say I devise some way of travelling faster than light( never going to happen), does this mean that I would also shrink to nothing? Does it mean if time stopped for me would everything else also stop? Including the time in which “GOD” exists in?

  29. Oli West says:

    Surely, if he was travelling near the speed of light and he threw a tennis ball at someone, surely it would be neither him nor the tennis ball that arrived first, but the shockwave from the massive nuclear reaction exploding out of our ‘near light speed’ man as the protons he hits fuse with his face and the energy from it destroys everything in his path. Our speedy man would be fine, but our poor lad waiting to catch the ball would be in a spot of trouble and umm… obliterated.

  30. Chris z says:

    @ed This is actually a very important theory, the fact that time itself changes when moving fast relative to an observer is actually proven every day by GPS satellites. If it weren’t for our understanding of time changes at high velocity, GPS accuracy would degrade very quickly since they depend on very accurate time and they are moving so fast.

    Any time the large Hadron collider is used relativistic effects need to be accounted for. Some particals such as muons have a very short half life in most cases (0. 9c). These experiments lead to important information for scientific advancement (discovery of new bosons) and would be difficult or impossible if these effects weren’t known.

    Without these studies certain theories about items in space would not be consistent either, for example the calculation of spin velocity of pulsar that are moving away or towards us at a very high rate of speed (detected via redshift, which is also a kind of effect related to this phenomenon).

    In the future, it is not unreasonable for high speed craft to become available to us, and high relativistic speed is easier to accomplish in space (the voyager craft are moving quite fast, over 3 times the distance from Earth to sun every year, and have lost around 11 seconds compared to earth from their perspective. They are powered by technology of 30 years ago, now new tech like ion propulsion mean we could accelerate to high speed over a long time period.)

  31. Ed says:

    Why do we mislead people, time is only affected by gravity or acceleration.
    GPS time is different because the gravity is different up there.

  32. tj says:

    Hi. I was actually just trying to fall asleep when I pondered upon the question of lightspeed travel. I know that even if you travel in such speed, it would still take millions or thousands of years to reach other galaxies. Putting aside every thing that’s logical, shall we? let’s ptetend for a moment that it IS possible… Would growth slow down during such travel? (Thus making it possible for us to reach other planets in a relatively reasonable age enough to be able to actually even explore that planet?) (Don’t sass me. I’m just curious… and possibly wish to write a story…) I know little about math or physics, so I would much appreciate a reply no matter how fictitious the situation may be.

  33. The Physicist The Physicist says:

    Pretty much. There’s a post here that talks about that a bit.

  34. jason says:

    So if a ship powered by dark matter was able to go light speed. It could not be a named mission cause human body could not take the mass increase. A unmaned mission would be impossible because we would not have computers fast enough. How can one reduce the mass increase while still picking up speed.

  35. David C. says:

    Loved this article, just discovered this website and I’m flabbergasted.
    An undoubtedly stupid question: if objects shrink in size relative to us when we travel close to light speed, and there’s symmetry in the equations (that’s my second question, probably already been asked somewhere in the comments but I had to start writing my questions or I would have got bored and played video games), does that mean if we are “stationary” and an object is moving fast relative to us, that it would appear to have infinite density? And aren’t there only two things we know about that might have infinite density – the big bang singularity, and black holes (which by the way, differ how?) So would this phenomenon of velocity affecting relative density play a role in explaining black holes, or at least dark matter (which if I understand correctly is an attempt to correct for matter that scientists assume must exist, given the observations of far-off stellar interactions?)? But I’m sure that’s probably the first thing taken into account.
    Oh yeah, the symmetry. So, how the fuck would that work with people? You can’t both be younger/older. Can you? I guess you could be both with a multiverse. But we’re not talking about real reverse time travel with relativity, so you don’t need a multiverse to explain the paradoxes, as done in Anathem and the Star Trek reboot. So but anyway, I’ve always thought the person who is traveling at a faster speed (and therefore moving through space-time, which means moving faster through time) would arrive at the later date faster, in less time, and therefore be younger. True or false?

  36. Rob says:

    If you were in a vessel that used a form of anti gravity device to propel it, or even a dark matter drive and you were travelling at the speed of light, how would it affect:

    * your speed
    * the device that propelled you, and

    and how does the speed of light affect the constituents of gravity and its pull?

  37. MUHAMMAD TALAL says:


  38. vargheese k j says:

    we should become energy or something equal to light as a form of energy to travel in the speed of light. because such a speed causes a complete molecular or atomic contraction in any body that moves towards a point. energy can not be destroyed but anything other than energy can be destroyed. if we are able to change ourselves as some kind of energy, it may be possible to travel in the speed of light. the biological clock within the body will stop ticking as the moving body’s value becomes zero when nearing the light speed. the body thus will not be affected by time or will not become aged.. because age will not affect the dead. :-)

  39. RAJAN ANAND says:

    if a object is moving at a speed of light,then how it is time travelled?

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